Receiver for bistatic doppler radar network

ABSTRACT

A multiple Doppler radar network can be constructed using only one, traditional, transmitting pencil-beam radar and one or more passive, non-transmitting receiving sites. Radiation scattered from the pencil beam of the transmitting radar as it penetrates weather targets can be detected at the receive-only sites as well as at the transmitter. In a bistatic system, the location of targets in Cartesian space can be calculated from the pointing angle of the transmitting antenna and the time between transmission of a radar pulse from the transmitter and detection at a passive receiver site.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to applications titled "BISTATICMULTIPLE-DOPPLER RADAR NETWORK", "WIDE-ANGLE MULTIPLE-DOPPLER RADARNETWORK", "RECEIVER ANTENNA FOR BISTATIC MULTPLE-DOPPLER RADAR NETWORK",all of which are filed on the same date as this application, thedescriptions of these related applications being incorporated byreference into the present specification.

FIELD OF THE INVENTION

This invention relates to weather radar systems and, in particular, to asystem that uses a plurality of passive, inexpensive receivers locatedat sites remote from the transmitter to collect data that can be used toidentify the presence and locus of two-dimensional and three-dimensionalwind fields in a predetermined volume of space.

PROBLEM

It is a problem in weather radar systems to implement an inexpensivesystem that collects sufficient data to provide accurate weatherinformation to the users. Existing weather radar systems that make useof a single Doppler radar transmitter/receiver produce data indicativeof only a radial component of the wind field. The collected radialcomponent data is then used to identify the presence and locus ofmeteorological phenomena extant in the volume of space that is scannedby the Doppler radar beam. These radial component measurements do notpresent a highly accurate picture of the weather in the monitored volumesince the radial Doppler radar beam cannot measure non-radial componentsof the wind.

Presently, in order to directly retrieve measurements of the magnitudeand direction of three-dimensional winds, at least two, and preferablymore, transmitting Doppler radar transceivers must be employed. Thereare severe limitations with this approach. First, the Doppler radartransceivers cannot possibly simultaneously scan the same region ofspace throughout a storm volume. Radial velocity measurements from thevarious radars are therefore taken at different times, and wind fieldsyntheses are contaminated by storm evolution and advection. Second,Doppler radars, with rotating high gain antennas, transmitters, andoperators are expensive to build, maintain, and operate. Thus,opportunities for multiple Doppler and overdetermined multiple Dopplerwind retrievals are fairly rare. This cost, and the difficulty incoordinating the operation of spatially distant Doppler radar sites haveprecluded the availability of three-dimensional wind fields as anoperational product for use in forecasting and warning of severeweather, initializing mesoscale models, or protecting weather sensitivesites such as airports.

It is impossible for each radar in a monostatic non-rapid-scantransmitting network to sample all locations in space concurrently. Scanstrategies can be established so that certain individual regions areexamined by three or more radars nearly simultaneously. Yet theconstraints of geometry prevent this from being extended to largeregions of space unless scanning proceeds in an extremely slow manner.Typically, during coordinated scanning, the difference in sampling timenear the ground is small. Unfortunately, differences can grow to threeminutes or more aloft. In convective environments, significant stormfeature evolution can occur between successive measurements of the samespace. Windfield syntheses based on such data are suspect. Volumeupdates rates in a multiple-Doppler radar network are limited by themost slowly scanning radar. Consequently, rapid-scanning and otherexotic and expensive techniques cannot escape this difficulty unless allradars in a network use new methods and/or technology.

An alternative to these monostatic Doppler radar systems is the Dopplerradar system which uses a single Doppler radar transmitter inconjunction with a remotely located scanning receiver to obtain winddata within a predefined volume of space. The Doppler radar transmittertransmits a "pencil beam" radar signal which is reflected and scatteredby targets located in the predetermined volume. The Doppler radar beamscans the predetermined volume in a defined scan pattern. A radialcomponent of the Doppler radar beam is reflected by targets, with thereturn (backscattered) signal being received at the scanning radarantenna. Another reflected component of the radar beam is received atthe remotely located radar receiver, which has an antenna that scans thepredetermined volume in coordination with the scanning radar beam. Thetwo received radar signals are used to construct two-dimensional and/orthree-dimensional wind fields. A difficulty with this system is that thecoordination of the operation of two scanning antennas and the necessarytiming coherence is difficult if not impossible to attain.

There is presently no Doppler radar system that can inexpensively andaccurately identify the presence and locus of meteorological phenomenain a predetermined volume of space. Low cost is obtained by sacrificingaccuracy. Accuracy of measurements can presently be obtained only by theuse of multiple Doppler radar transmitter/receiver installations, eachsite of which is expensive to implement and operate.

SOLUTION

The above-described problems are solved and a technical advance achievedin the field by the receiver for a bistatic multiple Doppler radarnetwork of the present invention. A multiple-Doppler radar network canbe constructed using only one, traditional, transmitting pencil-beamradar and one or more passive, low-gain, non-transmitting receivingsites. Radiation scattered from the pencil beam of the transmittingradar as it penetrates weather targets can be detected at thereceive-only sites as well as at the transmitter. In this bistaticsystem, the location of targets in Cartesian space can be calculatedfrom data that indicates the locations of the transmitting and receivingantennas, the pointing angle of the transmitting antenna and the timebetween transmission of a radar pulse from the transmitter and detectionat a passive receiver site. Surfaces of constant delay time formellipsoids with loci at the transmitter and receiver sites. To detectthe velocity of the targets, the echo signals are analyzed for Dopplershift. In a multiple receiver system, the determined meteorologicalphenomena presence and locus data from each transmitter-receiver pairare combined to accurately identify the location of the target. There isonly one result that translates to the data determined at each receiverand then a unique solution is obtained from the collected data.

The simultaneity of the sampling of individual resolution volumesreduces errors associated with storm evolution. It remains true thatevolution occurs during the time associated with a complete volume scanby the transmitter, and in a rapidly evolving convective situation thiscould be significant. Bistatic networks are, however, uniquely suited totake advantage of new rapid-scanning techniques and phased arraytransmitters. In a bistatic network, only one expensive radartransmitter need be installed and operated in order to achieve rapidupdates of full three-dimensional vector winds in complete volumes.

The extremely low cost of passive bistatic receiving sites and theaccuracy of the resultant data makes bistatic networks very attractivewhen compared to traditional monostatic networks. For the equivalentexpense of much less than two monostatic radar transmitters,weather-sensitive sites such as airports can be provided with fullthree-dimensional vector winds from a one transmitter-ten receiversystem. This affordability makes bistatic radar networks practical for awider range of scientific studies. Aircraft wake vortices of arbitraryorientation could be detected through the use of the combination ofupward-looking bistatic receivers and monostatic transmitter/receivers.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates in block diagram form the overall architecture of abistatic radar network;

FIG. 2 illustrates a set of constant delay time surfaces that are foundby the radar beam around a transmitter site and receiver site pair;

FIG. 3 illustrates the path geometry for directly transmitted andreflected radar pulses;

FIG. 4 illustrates in block diagram form additional details of thebistatic radar network;

FIG. 5 illustrates in block diagram form the architecture of a bistaticradar receiver;

FIG. 6 illustrates a perspective view of a typical passive bistaticradar network receiver antenna;

FIG. 7 illustrates the antenna sensitivity pattern for the antenna ofFIG. 6;

FIG. 8 illustrates in block diagram form the basic architecture of aprocessing element used in the bistatic network; and

FIG. 9 illustrates modifications made to the radar transmitter.

DETAILED DESCRIPTION

A multiple-Doppler radar network can be constructed using only one,traditional, transmitting pencil-beam radar and one or more passive,low-gain, non-transmitting receiving sites. Radiation scattered from thepencil beam of the transmitting radar as it penetrates weather targetscan be detected at the receive-only sites as well as at the activetransmitter. The Doppler Shifts of the radiation received at all thesites can be used to construct vector windfields in a manner similar tothat used with traditional radar networks. In a bistatic system, thelocation of targets in Cartesian space can be calculated from locationof the transmitter and receiver antennas, the time-tagged pointing angleof the transmitting antenna and the time between transmission from theactive radar and detection at the passive receiver site. Surfaces ofconstant delay time form ellipsoids with foci at the transmitter andreceiver sites. The echo time samples may be correlated to a positionalong the beam P if the receiving station R is synchronized with thetransmitter T and has the pointing angle of the transmitter antenna Aand the distance between the transmitter antenna A and the receiverantennas. The pointing angle and the location of the transmitter Tdefine the beam path P, O through the sky relative to the receiverantennas. If the receiver is then time synchronized with thetransmission of pulses from the transmitter T and knows the distancebetween transmitter and receiver antennas, the receiver can thendetermine the location along beam path P, O where each time sample ofthe echo signal originated.

There are scientific advantages particular to a bistaticmultiple-Doppler network: 1. Radial velocity measurements fromindividual pulse volumes are collected simultaneously at all thereceivers since there is only one source of radiation. 2. The intensityof the obliquely scattered radiation can be compared to Rayleighscattering predictions and used for hail detection. 3. Rapid scanning oflocalized weather phenomena can be aided by elimination of the need toscan with multiple scanning radars. This type of radar network also hassignificant economic advantages. Passive receiver sites contain no highvoltage transmitting equipment or large rotating antennas, and they alsorequire no operators and much less maintenance than rotating antennas.

There are shortcomings particular to bistatic radar networks: 1. Passivereceiving sites have lower gain antennas (broader field of view) thanscanning radar systems and are more sensitive to contamination fromtransmitter side-lobes and to secondary, or multiple scattering fromweather echoes. 2. Lower gain receiving sites are less sensitive to weakweather echoes. 3. Cartesian (u,v,w) windfields derived from bistaticnetwork data exhibit greater error components than those constructedfrom data from traditional monostatic networks containing equal numbersof radars when advection/evolution are not taken into account. Multiplescattering and side-lobe contamination levels are acceptable in mostsituations and can be reduced by the use of higher gain (narrower fieldof view) receiving antennas. The reduced sensitivity and increased errorcomponents of the bistatic network can be ameliorated by the use ofmultiple passive receiver sites, a practical solution due to their verylow cost.

System Architecture

The basic architecture of a bistatic multiple-Doppler radar network isillustrated in block diagram form in FIG. 1. The system includes atleast one pulsed Doppler radar transmitter T and a plurality ofreceivers R1-R5, at least one of which is a passive, inexpensivereceiver. The pulsed Doppler radar transmitter T generates a"pencil-beam" radar signal that is output, via a highly directionalscanning antenna A, into a predetermined volume of space V which may belocated in close proximity to or around the Doppler radar transmitter T.The scan pattern of the antenna A can either be the full volume of spacearound transmitter T or can be selected as a function of the topographyaround the Doppler radar transmitter site and the volume of interest V.The Doppler radar transmitter T can be of conventional design, such asthe NEXRAD WSR88D or NCAR's CP-2 or other well-known scanning pulsedDoppler radar systems. The transmitted radar beam P, consisting of aseries of radio frequency pulses, is reflected off targets (scatterers)ST located in the predetermined volume V. One component B of thereflected radar beam (backscattered) is received by the scanning antennaA of the Doppler radar transmitter T, while other components O of thereflected radar beam (obliquely scattered) are detected by the passivereceivers R1-R5.

As illustrated in FIG. 1, the volume of interest V is sited above andaround an airport AP so that meteorological phenomena BS in this volumeV that may impact airport operations can be detected. The radartransmitter T is typically located at a site somewhat distant from theairport AP so that the scan pattern of the scanning radar antenna A isreduced from a full semi-spherical pattern to a narrow segment of asphere. This reduced scan pattern enables the radar beam P to morefrequently probe the volume V. The transmitted radar beam P that isscattered off various targets ST in the volume V must be detected by anumber of receivers R1-R5 in order to obtain an accurate determinationof the three-dimensional motion of the meteorological phenomena. Asingle receiver obtains only partial information since the radiallytransmitted beam ST cannot identify any motion components that areorthogonal to the radial direction of transmission. The passivereceivers R1-R5 are located at sites remote from the Doppler radartransmitter T site. There are typically a plurality of passive receiversR1∝R5 associated with a Doppler radar transmitter T to provide goodareal coverage and to resolve the limited information obtained from theradial radar beam ST. Their siting is a function of topography of theregion and the accuracy of the measurements that are desired.

In a bistatic Doppler radar system, the location of a scatterer ST inCartesian space can be calculated from the time-tagged pointing angle ofthe transmitting antenna A and the time between transmission of a radarpulse from the radar transmitter T and detection of the reflected radarpulse at the passive receiver site. Surfaces of constant delay time formellipsoids with foci at the transmitter site and the receiver site, asillustrated in cross-section view in FIG. 2 for a short-baseline system.The location of a scattering particle ST in the volume V, relative tothe transmitter site, is specified by: ##EQU1## where p is the angleenclosed by the transmitter-particle and transmitter-receiver vectors, Cis the speed of light, D is the distance between the transmitter siteand the receiver site, and a and e are the azimuth and elevation anglesrelative to the transmitting radar antenna A. This formulation isillustrated schematically in FIG. 3 for the two-dimensional case (y=0).In this simplified case the angle p is equal to the elevation angle ofthe transmitter antenna A.

In bistatic radar systems, the length of effective radar resolutionvolumes [roughly proportional to ∂ (range along the transmitted beams)/∂(delay time)] are not constant. This can be seen easily by contrastingthe arrival time of radiation scattered from transmitted beams Pdirected toward and away from the bistatic receiver along thetransmitter-receiver baseline. All radiation scattered forward towardthe receiver site arrives at the receiver simultaneously, regardless ofthe scatterers' locations along the beam P (all pathlengths areidentical). In contrast, radiation backscattered toward the Dopplerradar transceiver T is delayed by one microsecond for each 150 meterchange in scatterer placement along the baseline. In this case, theeffective length of resolution volumes is the same as that for atraditional monostatic weather radar. The same result occurs if thescatterers ST are off the baseline but are extremely distant from thebistatic radar network. At most transmission angles, near the bistaticcouplet, but away from its baseline, the resolution-volume length isexpanded by factors ranging from 1 to 4.

The reflectivity-weighted particle velocity can be retrieved at anypoint by solving the system of equations:

    VR.sub.i =u sin (a.sub.i) cos (e.sub.i)+v cos (a.sub.i) cos (e.sub.i)+w.sub.p sin (e.sub.i), i=1, n                   (2)

where VR_(i) are the radial velocities measured by the n radars, a_(i)and e_(i) are the azimuth and elevation angles of the n radars, and u,v, w_(p) are the Cartesian components of the particle velocity field. Indual-Doppler analyses the vertical air-parcel velocity w_(a) is obtainedthrough the integration of mass continuity. If measurements areavailable from more than three radars, and a reflectivity-terminalvelocity relationship is assumed, the system of equations isoverdetermined and can be solved by minimizing error.

Bistatic Wind Field Synthesis Equations

Equation (2) can be modified to apply to a Doppler network consisting ofone radar transmitter T and one or more passive bistatic receivers R1-R5as shown by the matrix equations illustrated in Appendix A. In AppendixA, VR_(i) are the particle velocities perpendicular to the ellipsoidalconstant phase surfaces calculated from the Doppler-shifted radiation atthe n bistatic receiver sites R1- R5, a_(i) and e_(i) are the azimuthand elevation angles of the illuminated volume relative to the n passivereceiving sites, VR_(t) is the radial velocity calculated at thetransmitting radar T, and a_(t) and e_(t) are the time-tagged pointingangles of the transmitting antenna A. Equation (3) for a three-receivernetwork can be solved for (u, v, w_(p)) as shown in Appendix B. The overdetermined cases can be solved similarly by minimizing error.

Using the location of both the transmitter antenna A and receiverantenna S, the location of the target as defined by these variables canbe determined when the radar echoes are received.

Accuracy

The variance and standard deviations of the estimates of (u, v, w_(p))in the three-radar network are expressed by: ##EQU2## Typical standarddeviations in bistatic radar networks are twice that of monostatic radarnetworks consisting of comparable numbers of radars. Since the cost ofbistatic radar receiving sites is very low, less than one-thirtieth ofthat of conventional transmitters, it is practical to deploy manypassive receivers in a typical bistatic radar network. In this fashion,accurate wind fields can be retrieved at a much lower cost than withtraditional systems. A ten receiver bistatic radar network providescomparatively accurate wind field synthesis. The vertical particlevelocities are determined accurately down to elevations below 2 km,allowing mid-level and low-level boundary conditions to be applied tothe downward integration of mass continuity, thus avoiding commonproblems associated with the establishment of ground-level boundaryconditions. The cost of such a radar network is less than that of a twomonostatic transmitter radar network.

In the Rayleigh limit, the intensity of the radiation scatteredobliquely from a transmitted radar beam varies with the scattering anglemeasured relative to the transmitted E vector such that I∝I₀ sin² (θ),where I is the intensity of the scattered radiation, I₀ is the intensityof the incident radiation, and θ is the angle between the incident Evector and the propagation vector of the scattered radiation. Inaddition, the bistatic geometry affects the size and shape of theresolution volumes as noted above. Therefore, modified versions of thetraditional radar equation must be used. This angle-dependent scatteringintensity strongly impacts the usefulness of bistatic radars and thechoice of appropriate transmitter and receiver sites. If horizontallypolarized radiation is transmitted, then there is a circular region atground level from which there is very little scattering toward aparticular receiver. This region is the locus of all points from whichthe E vector of transmitted beams points at the receiver.

The use of vertical polarization in the radar beam P moves thelow-sensitivity notch from the ground level to a vertical plane over thetransmitter-receiver baseline. Bistatic systems that employ circularpolarization have no low-sensitivity notch. While not nearly assensitive as a traditional monostatic weather radar, the displayedbistatic configuration, using vertically polarized transmissions,provides a minimum sensitivity of 0-10 dBZ within the usefulmultiple-Doppler lobes and below -5 dBZ within 4 km of the receivingsite. This is adequate for most purposes, but applications that requireextreme sensitivity need to use higher-gain receiving antennas, say 25dB, shorter transmitter-receiver baselines, or multiple receiverconfigurations to achieve sensitivities below -15 dBZ.

Frequency Coherence

In order to accurately determine the velocities perpendicular to thebistatic radar network's ellipsoidal delay time surfaces from thereflected radar pulses received at the remote bistatic receivers R1-R5,extremely accurate knowledge of the frequency of the transmitted radarpulses must be available. Errors of just 3 Hz result in velocity errorsof approximately 0.15 m s⁻¹ (assuming 0.1-meter transmissions andignoring the expansion of the frequency-velocity relationship near thetransmitter-receiver baselines). This corresponds to a relativefrequency error of only one part in 10⁹. Traditional radar frequencysources, while extremely stable over typical transmit-receive delaytimes, may drift by much more than this over the longer term. Solutionsto this frequency coherence problem include the use of atomic frequencystandards at the transmitter T and the receivers R1-R5, direct detectionof the frequency of the transmitted pulses through sidelobe coupling orsignals sent to the receivers R1-R5 through cables or the atmosphere,and the use of one common frequency standard for the transmitter T andreceivers R1-R5.

Timing Coherence

In order to determine the location of resolution volumes accurately, theprecise time of the transmission of the radar pulses from transmitter Tmust be known at all of the receivers R1-R5. To achieve suitableaccuracy, this timing must be known within approximately 100 ns. This isparticularly true near the transmitter-receiver baselines whereeffective resolution volumes are expanded.

There are several approaches to meeting this timing coherencerequirement, falling into two main categories. In the first category,extremely accurate time is kept at both the transmitter T and receiversR1-R5 and data indicative of the pulse transmission time from thetransmitter T is sent to all the receivers R1-R5. The informationarrives at each receiver R1-R5 well after the obliquely scatteredradiation O, but the pulse repetition frequency can be used to correlatethe received radiation O with the pulse transmission time. In the secondmethod, the transmitted pulse is detected directly at the receiversR1-R5. This direct radiation, from the existing sidelobes or throughradiation beamed intentionally at the remote receiving antennas, alwaysarrives before any scattered radiation and can be used to start aranging clock.

Atomic clocks can provide extreme timing accuracy but drift relative toeach other. Even though accurate within 1 part in 10¹², they tend todrift apart by roughly 100 ns per day and these clocks must berecalibrated frequently. Either as a method of recalibration or as anindependent timing method, the arrival time of direct-path radiationfrom the sidelobes of the transmitter antenna A could be measured. Thisradiation may be difficult to detect in sheltered locations, thuscomplicating the accurate calculation of its arrival time.

The preferred method of achieving both timing and frequency coherence isto link the transmitter T and all the receivers R1-R5 to an externaltiming standard. Both Loran and Global Positioning Satellite (GPS)signals can provide the needed information, but only the GPS signalsinclude time of day information so that the timing coherence can alwaysbe maintained without recalibration. Both signals can be used to achievefrequency coherence to well within one part in 10¹⁰ (0.3 Hz at λ=0.1 m)if disciplined oscillators with high short-term stability are used.

Bistatic Radar Network Implementation

FIG. 4 illustrates a more detailed implementation of the bistatic radarnetwork. For simplicity of description, only one passive receiver R isillustrated. Pulsed Doppler radar transmitter T scans a predefinedvolume of space using a stream of radar pulses P transmitted in a radialdirection from the antenna A into the volume V. The antenna A follows aprecisely controlled scan pattern to sweep all points in the predefinedvolume V with the radar beam P on a periodic basis. One component B ofthe radar beam P is backscattered from a scatterer ST to antenna A alongthe transmit path while a second component O of the transmitted radarbeam P is reflected at an oblique angle from the scatterer ST towardreceiver R. Receiver R, being a passive element and not steerable, mustdetect the obliquely reflected component 0 and reject background noiseas well as signals arriving from outside of the predefined volume V,and/or from regions of the predefined volume V that are not monitored byantenna S. In order to enhance the performance of receiver R, adirectional antenna S is used to receive signals from only a segment ofspace, which segment includes all or a portion of the predefined volumeV. An excellent choice for the antenna S is a slotted waveguide antenna,which exhibits high gain in a predetermined direction and significantlylower gain elsewhere. The slots are arranged to produce a desired gainpattern which falls off steeply outside of the desired field of view.

The signals received by antenna S consist of base frequency signals (forexample 2809 MHz) with the superposition Doppler offset componentsrelated to the collective movement of the distributed targetsilluminated by transmitter T at a particular location in space. Thesereceived signals are coupled to bistatic receiver 411 which issynchronized to the transmitter frequency and pulse transmission timefor accuracy of ranging and gating. A precisely controlled oscillator412 is used as a reference frequency source for bistatic receiver 411.The output frequency of oscillator 412 is controlled by processor 413which receives reference signals from a number of sources. Timingantenna GA receives timing signals from a source G that is common to allreceivers R1-R5 in the bistatic network. An example of such a commontiming source G is the Global Positioning Satellites (GPS), whichtransmit signals of precise frequency. These signals are also timestamped. The received GPS signals are used by processor 413 todiscipline oscillator 412 to maintain frequency coherence with the GPSreference, which in turn allows synthesis of signals coherent withtransmitter T. The received GPS signals can also be used as a timereference to obtain synchronization with the transmitted radar pulses. Acontrol processor 423 located at the transmitter T also receives GPStiming information and, in the particular example shown, disciplinestiming reference oscillator 422 in identical fashion. It also generatesdata indicative of the time delay between the radar pulse and the GPStiming signals which occur at 1 pulse per second. This data istransmitted to the receiver R via a data link D and used at the receiverR to obtain timing synchronization with the radar pulses. Alternatively,the transmitter T can transmit pulses directly to receiver R inconjunction with the scan beam. In either case, the reference frequencyoutput by oscillator 412 and timing synchronization information fromprocessor 413 are used by bistatic receiver 411 to produce receivedpulse data I, Q indicative of the in-phase and quadrature components ofthe received radar echoes. This data and the timing data indicative ofthe time difference between the transmitted pulse and the receivedsignals represent receiver data that are indicative of the locus of thescatterers. This data is stored in processor 413 and retrieved on aperiodic basis by central processor 431, which uses this retrieved datafrom all the receivers and the time-tagged antenna pointing data toaccurately identify the locus of the scatterers. Alternatively,processing can be distributed and processor 413 can perform many of thecomputation functions described below as performed by central processor431.

In this particular example, timing antenna TA receives timing signalsfrom a source that is common to all receivers R1-R5 in the bistaticnetwork. An example of such a source is the GPS satellites, whichtransmit signals of precise frequency. These signals are also timestamped. The received GPS signals are used by processor 423 todiscipline oscillator 422 to maintain frequency and timing coherencewith the remainder of the bistatic network. The reference frequencyoutput by oscillator 422 and timing synchronization information fromprocessor 423 are used by transceiver 421 to produce the frequencycoordinated transmit pulse and received radar echo data I, O, indicativeof the in-phase and quadrature components of the received signals. Inaddition, antenna A outputs antenna pointing angle information regardingthe precise direction that the transmitted radar beam P is output byantenna A. This data is stored in processor 423 and retrieved on aperiodic basis by central processor 431. Alternatively, processing canbe distributed and processor 423 can perform many of the computationfunctions performed by central processor 431.

The central processor 431 can be connected via data links to all thereceivers R1-R5 and transmitter T that form the bistatic network. Thedata received from these elements represent the information that definesthe path of the transmitted radar beam as well as the paths that thereceived reflected components traversed. The timing informationassociated with each of the received signals are indicative of thedistance of the target from the receivers R1-R5 and transmitter T. Thisdata is used by central processor 431 to compute the locus and motionvectors for the detected target.

Slotted Waveguide Antenna

The slotted waveguide antenna disclosed as the preferred embodimentherein consists of a length of waveguide that is constructed toimplement a multi-element antenna which produces a focused receiverpattern to receive signals from only a segment of space (controlledfield of view), rather than, for example, an omnidirectional antennawhich receives signals from all directions without preference.Slot-antenna arrays have been used in many ground-based and airborneradar systems. Waveguide-fed slot-antenna arrays are used as resonantand travelling wave antennas when precise amplitude and phase controlare needed.

The slot is a commonly used radiator in antenna systems. The slot can beincorporated into the antenna feed system, such as a waveguide orstripline system without requiring a special matching network.Low-profile high gain antennas can be easily configured using slotradiators, although their inherent narrow frequency bandwidth can limitantenna performance in some applications. A slot cut into the waveguidewall which interrupts the flow of currents couples power from thewaveguide modal field into free space. A singly moded waveguide isnormally used for a slotted waveguide array design, and the spacingand/or orientation of the slots along the edge of the waveguide are usedin order to control aperture illumination. A travelling waveguide slotarray has five significant characteristics: the resonant slots arespaced by either more or less than one-half of the waveguide wavelength;the slotted waveguide should be terminated by a matched load; all slotsin the array are resonant at the center frequency; the beam is offbroadside and is frequency dependent; array efficiency is less thanunity. Travelling wave slot antenna arrays are either uniformly spacedarrays to produce a low sidelobe pencil beam or nonuniformly spacedarrays to produce shaped beam patterns.

For the purpose of the preferred embodiment of the bistatic network, theantenna criteria are: a (vertically) polarized beam to match thetransmitter radar polarization, a beam pattern that exhibits a sharpreduction in gian for elevation angles above and below the desired fieldof view, a broad azimuthal pattern and relatively low sidelobes. Apreferred pattern has sidelobes of below 20 dB from the horizon down tobelow the ground, a flat top pattern from the horizon to some selectedelevation angle, sidelobes below 20 dB from this elevation angle up tozenith. As an example, too implement this antenna, using a slottedwaveguide, a 4 meter long element was constructed using 58 uniformlyspaced slots machined therein. The resultant gain is 13 dB and theantenna exhibits an elevation beam width of 20 degrees and an azimuthalbeamwidth of approximately 160 degrees. FIG. 7 illustrates the beampattern of such an antenna. An alternate design, illustrated in FIG. 6,having higher gain can achieve a 4 degree elevation pattern and usesreflectors RF1, RF2 placed on both sides of the waveguide W reduce theazimuth beam pattern to approximately 40 degrees. The implementationillustrated in FIG. 6 is a top fed antenna, so the waveguide FD issimply the input power feed and the segment W represents the activeelement that contains the precisely machined slots to produce thedesired antenna pattern. The segment W of the antenna is terminated by aload L. A support SP is provided for mounting segment W, with itsreflectors RF1 & RF2, and waveguide FD in the proper position andattitude.

Bistatic Receiver

FIG. 5 illustrates additional details of receiver R. Bistatic receiver411, details of which are shown in this figure, is connected to aprecision frequence reference oscillator 412, such as an ovenized,highly stable 10 MHz voltage controlled oscillator. The output signalfrom the oscillator 412 is passed through a bandpass filter 513, losselement 514 to frequency distribution circuit 516, which passes onecomponent of the signal to frequency distribution circuit 516. Thefrequency distribution circuit 515 in conjunction with loss element 517,and RF synthesizer 518 are used to generate frequency locked 15 MHzoutput frequency, and a synthesized output (in this implementation 2869MHz) frequencies using circuitry that is analogous to that used in radartransmitter T. Frequency distribution circuit 515, in conjunction withIF generator 519, is used to generate a frequency locked 60 MHz signal.The 10 MHz output from oscillator 412 and a trigger pulse from processor413 are used by Pulse Generator 520 to produce a test pulse for use forreceiver calibration by directional coupler circuit 521. The receivedsignals from antenna S are fed through interface TR designed to blockstrong signals of amplitude above the damage threshold of amplifier 522.Coupler 521 allows the test signal to be coupled (with low loss) intoreceiver R. The signal is then sent into bandpass filter 523. Thebandpass filter 523 ensures that only the signal components of interestare passed to mixer circuit 524. The 2869 MHz signal output by RFsynthesizer 518 is passed through bandpass filter 525, loss element 526and applied to mixer circuit 524. The 2869 MHz signal is mixed with thereceived 2809 MHz signal and the resultant 60 MHz signal applied viaamplifier 527 and bandpass filter 528 to quadrature detector 529 whichoutputs signals indicative of the in-phase and quadrature components ofthe received signal on leads I and Q, respectively.

Transmitter Modifications

There are several methods to coordinate the operation of the transmitterT and receivers R1-R5. One possibility is to place a low gain, precisiontime base receiver in the vicinity of transmitter T to monitor thefrequency of operation of transmitter T. To reduce bandwidthrequirements, only the difference from some monimal frequency can besent. This data is then transmitted to the receivers R1-R5 of thebistatic radar network.

An alternative method of coordination is to modify an existing radartransmitter T. FIG. 9 illustrates modifications made to a typical radartransmitter, such as the NCAR CP-2 system, that accommodate therequirements of the bistatic radar network. The transceiver incorporatesnew frequency generation apparatus, a GPS receiver, modem and apparatusthat acquires and time tags antenna pointing angle data from the antennaA. FIG. 9 illustrates the new frequency generation apparatus thatconsists of a highly stable oscillator 911, such as an ovenized highlystable 10 MHz voltage controlled oscillator. A control voltage obtainedfrom processor 423 (FIG. 4 and element 828 on FIG. 8) synchronizes theoutput frequency of oscillator 911 with the transmitter frequency togenerate a base frequency reference for the RF, IF and gating clocks.The clock signal output by oscillator 911 is applied through bandpassfilter 912 and loss element 913 to isolation amplifier and power dividercircuit 914. One segment of the 10 MHz signal is converted by RFmultiplied circuit 915 to a 60 MHz signal which is transmitted to the IFsection of the CP-2 radar system. Another segment of the 10 MHz signalis applied to amplifier, multiplier, divider circuit 916 which generatesa 15 MHz component which is output to processor 423 and CP-2 radarsystem. Amplifier, multiplier, divider circuit 916 also outputs a 10 MHzsignal through loss element 917 to RF synthesizer 918 which produces a2869 MHz signal that is transmitted to the CP-2 radar system. Thefrequency selected represents stable local oscillator (STALO) theoperating frequency, 2809 MHz of the existing standard radar transmitterT which is obtained by mixing the 2869 MHz with 60 MHz and selecting thelower sideband for subsequent amplification and pulse gating.

Data Processor

FIG. 8 illustrates additional details of the processor structure that isused to convert the received data into meteorological phenomena locusand motion information. To simplify the description, the processor isdisclosed as performing the calculations in the distributed processingmode, where processor 413 (FIG. 4) receives the I and Q signals as wellas timing signals from bistatic receiver 411 and the GPS signals fromGPS antenna GA. Processor 413 is illustrated as embodied in a personalcomputer that consists of a processing element 811, bus 812 and aplurality of peripheral devices that are connected to bus 812. Acommunications interface 821 interfaces the bus 812 with a communicationlink to, for example, transceiver T and central processor 431. Two SCSIinterface cards 822, 823 provide memory devices 824, 825, respectively,with an interconnection to bus 812. Video card 826 provides a graphicalinterface to display 827. The remaining circuitry illustrated in FIG. 8comprises the timing and gating circuitry that is required for thereceived radar signal processing. GPS receive card 831 is interconnectedwith the GPS antenna GA to convert the received GPS signals into timingsignals indicative of the time stamped clock signals that are used tosynchronize each receiver in the bistatic radar network to the operationof the transmitter T to achieve the timing coherence. The GPS card 831outputs time signals to bus 812 and 1 pulse per second clock signals tocrystal discipline card 832 and timing synchronization card 833. Thecrystal discipline card 832 reads the GPS time from bus 812 and the 1pulse per second clock signal from GPS receive card 831 to produce thetiming synchronization data required to ensure that oscillator 412operates in both frequency and timing synchronization with thecorresponding oscillator 422 in transmitter T. The timingsynchronization card 833 obtains the 1 pulse per second GPS clock signalfrom the GPS receive card 831, GPS timing data from bus 812, transmittertiming data from bus 812 (obtained via data link D) and the 15 MHz clocksignal from bistatic receiver 411 (FIG. 4). Timing synchronization card833 uses the 15 MHz signal derived from the stable oscillator 412 toproduce a 7.5 MHz clock signal that is frequency locked with the 10 MHzoutput of oscillator 412. The 7.5 clock signals are triggered and gatedby a timing gate signal from timing synchronization card 833. Thesesignals are applied to signal converter circuit 834. A timing generator834a converts these two signals into gating triggers, synchronized withthe transmitted radar pulses, which gating triggers are used to inputthe in-phase and quadrature signals obtained from bistatic receiver 411into analog to digital converter circuit 834b. The digitized output ofanalog to digital converter circuit 834b is used by digital signalprocessor 834c to execute a pulse-pair algorithm that calculates thereal and imaginary components (A & B) of the lag one autocorrelations ofthe in-phase and quadrature (I & Q) signals received from bistaticreceiver 411 as well as the power (P) of the received radar echoes.These radar echoes are from obliquely scattered components of thetransmitted pulses for each gate of the transmitted beam. The computedvalues of A, B, P are transmitted via bus 812 to memory devices 824, 825for storage for later processing using the equations of Appendix B toidentify the locus and motion components of the detected meteorologicalphenomena BS. The stored data is compiled into files for transmission tothe central processor 431 which performs the final computations.

Central Processor

Central processor 431 periodically polls all receivers R1-R5 as well astransmitter T to obtain the corresponding radar echoes. The processingof the data from a plurality of receivers pursuant to the equationslisted in the Appendices is well-known and not described herein in theinterest of brevity of description. The data collection from each of thereceivers R1-R5 to central processor 431 can be done on a dial-uptelephone basis, dedicated data links or even RF link basis. Theserepresent matters of engineering choice and are best implemented on asite-specific basis.

Transmitter Antenna Pointing-Angle Information

Accurate, time-tagged transmitter antenna pointing-angle information isnecessary to calculate the location of scatterers ST in space. Asignificant characteristic of the bistatic network is that the use of acommon timing source G, such as GPS satellites, obviates the need forthe exchange of timing synchronization data between transmitter T andthe receivers R1-R5 on a pulse-by-pulse basis. The time stamped timinginformation received from the GPS satellites establishes a time baselinewhich can be used by each receiver R1-R5 to dynamically compute thepulse origination time of each radar pulse produced by transmitter T, aslong as the timing sequence remains invariant and each receiver R1-R5has data indicative of the pulse timing offset from the baseline timingsignals. Thus, transmitter T can broadcast pulse origination time offsetdata that indicates the origination time of a radar pulse with referenceto one of the predetermined baseline timing signals. Each receiver R1-R5can then use this received data and knowledge of the pulse repetitionfrequency of transmitter T to extrapolate the pulse origination time forsuccessive pulses output by transmitter T. Similarly, as long as antennaA follows a precisely determined antenna scan pattern, scan patternorigination data can be broadcast to receivers R1-R5 at reducedresolution still enabling these receivers R1-R5 to determine antennapointing information by extrapolation for each pulse output bytransmitter T. Thus, the baseline timing information available from acommon timing source enables all the receivers R1-R5 to operateindependent of the transmitter T, and yet stay in synchronization withthe pulses output by transmitter T. The primary source of errors in thissystem would be any deviation from the scan pattern or pulse repetitionfrequency due to transmitter anomalies. The concentration of these errorsources in the one manned site enables the system to be efficiently andrelatively inexpensively optimized. Furthermore, the addition ordeletion of receivers from the bistatic network becomes a matter ofsimple expediency, since the operationally independent receivers are notintimately linked to transmitter T. Central processor 431 can adjust theprocessing algorithms to account for the variation in the number ofsources of data on a dynamic basis.

SUMMARY

The bistatic network provides a number of improvements over existingweather radar systems. The receivers operate independent of thetransmitter by using a common time-tagged signal source. In order toaccomplish this there must be a precision common time base. The pulseorigination time and antenna pointing angle data can be derived at eachreceiver by reference to the precision time base and parameterinformation received from the transmitter. The receivers can beconfigured into a network in a manner that is substantially independentof the transmitter, and the number of receivers that are presentlyactive can be varied without extensive effort. In addition, thereceivers make use of an antenna that has a directional receive patternand are aligned to monitor at least a segment of the volume scanned bythe transmitted radar beam. By selection of the number, sites andreceive patterns of the receiver antennas, the precision of the datacollection within various regions of the volume of interest can beprecisely controlled.

I claim:
 1. A radar receiver for determining presence and locus ofscatterers in a predefined space, wherein a transmitter transmits afocused beam of high frequency energy into said predefined space in apredetermined scan pattern, with said beam comprising a series ofpulses, each having a pulse origination time as it is emanated from saidtransmitter, and wherein a timing source transmits deterministic timingsignals to said radar receiver, apparatus for synchronizing operation ofsaid receiver with said transmitter comprising:means receiving saiddeterministic timing signal, means, responsive to pulse originationscheduling data generated in said receiver exclusive of saidtransmitter, for determining pulse origination times of said pulsesemanating from said transmitter; and means, responsive to receipt ofcomponents of said transmitted beam that are reflected from scatterersin said predefined space and responsive to said determined pulseorigination times, for generating data indicative of locus of saidscatterers in said predefined space.
 2. The receiver of claim 1 whereinsaid generating means comprises:means, responsive to data received fromsaid transmitter that defines said scan pattern, for computing adirection from said transmitter that each pulse in said transmitted beamemanated from said transmitter.
 3. The receiver of claim 1 wherein saidtiming signal emanates from a single signal source whose output issubstantially concurrently receivable at said transmitting means andsaid at least one receiver.
 4. The receiver of claim 3 wherein saidsingle signal source is a satellite based transmitter.
 5. The receiverof claim 1 wherein said transmitter transmits data to said receiver toindicate a pulse offset time indicative of a time difference between aone of said deterministic timing signals and a pulse origination time ofone of said series of pulses, said pulse origination determining meanscomprises:means for computing said pulse origination time for each pulsein said series of pulses based on a pulse repetition rate of said seriesof pulses emanating from said transmitter, said pulse offset time andsaid deterministic timing signal.
 6. The receiver of claim 5 whereinsaid pulse origination determining means further comprises:means forregularly retrieving said pulse offset time data from said transmitter.7. The receiver of claim 1 wherein said apparatus furthercomprises:means for producing data, exclusive of said transmitter,indicative of a direction from said transmitter that said transmittedbeam emanated.
 8. The system of claim 7 further comprising:means forcomputing a scatterer locus from said component receipt data and saiddirection and time of origination data for each of a plurality of pulsetime intervals.
 9. The system of claim 7 further comprising:means forcomputing a scatterer velocity from said component receipt data and saiddirection and time of origination data from said transmitting means foreach of a plurality of pulse time intervals.
 10. A Doppler weather radarreceiver for determining the locus of meteorological phenomena in apredefined space, wherein a Doppler radar transmitter transmits afocused beam of high frequency energy into said predefined space in apredetermined scan pattern, with said beam comprising a series ofpulses, each having a pulse origination time as it is emanated from saidtransmitter, and wherein a timing source transmits deterministic timingsignals to said radar receiver, apparatus for synchronizing operation ofsaid receiver with said transmitter, comprising:means for receiving saiddeterministic timing signal to synchronize operation of said receiverwith said Doppler radar transmitter; means, responsive to pulseorigination scheduling data, for determining in said receiver exclusiveof said transmitter a pulse origination time for said pulses emanatingfrom said transmitter; and means, responsive to receipt of components ofsaid transmitted beam of pulses that are reflected from scattererscomprising features of said meteorological phenomena in said predefinedspace and responsive to said determined pulse origination time, forgenerating Doppler radar data indicative of locus of said scatterers insaid predefined space wherein said receiver is located at a site remotefrom said Doppler radar transmitter.
 11. The system of claim 10 furthercomprising:means for producing azimuth and elevation data indicative ofa direction from said Doppler radar transmitter that said transmittedbeam emanated.
 12. The system of claim 11 further comprising:means forcomputing a scatterer locus from said component receipt data and saiddirection and time of origination data for each of a plurality of pulsetime intervals.
 13. The system of claim 11 further comprising:means forcomputing a scatterer velocity from said component receipt data and saiddirection and time of origination data from said transmitting means foreach of a plurality of pulse time intervals.
 14. The receiver of claim10 wherein said generating means further comprises:means, responsive toinformation stored in a memory in said receiver that defines saidtransmitter beam scan pattern, for computing a direction from saidDoppler radar transmitter that each pulse in said transmitted beamemanated from said Doppler radar transmitter.
 15. The system of claim 10wherein said timing signal emanates from a single signal source whoseoutput is substantially concurrently receivable at said Doppler radartransmitter and said receiver.
 16. The system of claim 15 wherein saidsingle signal source is a satellite based transmitter.